EP0386894B1 - Focussing an electron beam - Google Patents

Focussing an electron beam Download PDF

Info

Publication number
EP0386894B1
EP0386894B1 EP90301563A EP90301563A EP0386894B1 EP 0386894 B1 EP0386894 B1 EP 0386894B1 EP 90301563 A EP90301563 A EP 90301563A EP 90301563 A EP90301563 A EP 90301563A EP 0386894 B1 EP0386894 B1 EP 0386894B1
Authority
EP
European Patent Office
Prior art keywords
optimum
focussing
investigation
specimen
magnifications
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP90301563A
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0386894A3 (en
EP0386894A2 (en
Inventor
Kimio Kanda
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hitachi Ltd
Original Assignee
Hitachi Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Hitachi Ltd filed Critical Hitachi Ltd
Publication of EP0386894A2 publication Critical patent/EP0386894A2/en
Publication of EP0386894A3 publication Critical patent/EP0386894A3/en
Application granted granted Critical
Publication of EP0386894B1 publication Critical patent/EP0386894B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/02—Details
    • H01J37/21—Means for adjusting the focus

Definitions

  • the present invention relates to a method of focusing an electron beam on a specimen in an automatic way and is particularly, but not exclusively, concerned with the application of such an automatic focussing method to an electron microscope.
  • the present invention also relates to a device for focussing an electron beam, to a method of operating an electron microscope, and to an electron microscope having an automatic focussing device.
  • Standard automatic focussing methods involve investigation of the optimum focus for a fixed magnification, which fixed magnification is determined by the operator, and which normally corresponds to the magnification at which the operator wishes to investigate a specimen.
  • the determination of the optimum focus at the fixed magnification is carried out on the basis of detection of secondary electrons emitted from the specimen when the electron beam of the electron microscope is incident on the specimen.
  • a suitable detector can detect such secondary electrons, and by investigating the variation in the secondary electrons as the electron beam is scanned across the specimen, the optimum focus can be determined.
  • the variation in the secondary electron signal is investigated for each of a plurality of current values, which current values are the values of the current controlling the electromagnetic condenser lens which determines the focussing of the electron beam. This way, a range of outputs is obtained, and the optimum one of those values can be selected.
  • the inventors of the present invention have appreciated that the sample itself may have an effect on the magnification at which optimum focussing can be achieved.
  • the magnification is determined by the ratio of the width of the picture generated, normally on a CRT (VDU) to the width of scanning of the electron beam. If the scan time is fixed, then for given magnification, the scan width is also fixed. If, over the scanning width, the specimen has a detailed surface structure, then a large variation will be detected in the output of secondary electrons and it is that large variation which is used to detect the focus. Therefore, if the sample has a detailed surface structure, higher magnifications will provide better focussing.
  • the present invention finds its origin in the realisation that the optimum magnification will vary with the sample, and the standard system, operating at fixed magnitudes, or the system of US 4199681, in which it was assumed that the best results are necessarily achieved at higher magnifications, are not satisfactory.
  • a method wherein the investigating of results of the investigation operations involves repeating step b) for each magnification or scanning time or speed of the plurality of magnifications or scanning times or speeds, thereby to determine a plurality of optimum values of the output signal, in which said plurality of optimum values correspond to the plurality of magnifications or scanning times or speeds.
  • a device for automatically focussing an electron beam on a specimen having means arranged to perform a plurality of investigation operations for a corresponding plurality of magnifications or scanning times or speeds of the beam on the specimen, each investigation operation involving the investigation of an output signal generated from the specimen which is irradiated by the beam with a plurality of foci of the beam characterised in that the device has means for investigating the results of the plurality of investigation operation, said means arranged to perform of plurality of the investigation operations including; first selection means for selection amongst the plurality of magnifications or scanning times or speeds; second selection means for selecting amongst the plurality of foci; output means for generating the output signal due to the focussing incidence of the beam on the specimen; and means for determining an optimum one of the plurality of magnifications or scanning time or speeds.
  • the focus is varied e.g. by varying the consender lens current in an electron microscope. Normally, this will be done as a series of steps, starting from a relatively low value. It is then desirable that the "height" of the steps is fixed, and also that the maximum and minimum values of the range over which the current is stepped is also fixed. This is because it is important that the system can adapt to a range of specimens. It should be noted that this fixing of the range and "height" of the current steps does not occur in US 4199681.
  • the height and range of the current steps may be fixed automatically. In some cases, however, it is desirable that at least the range be adjustable by the operator of the electron microscope.
  • the maximum and minimum values of the range of magnifications is also fixed or adjustable by the operator.
  • the operator may know from his experience that the likely optimum magnification for a particular specimen lies within a limited range of magnifications. Then, that range can be set, and therefore the method of the present invention need operate only over that limited range, thereby speeding up the determination of the optimum focussing current.
  • magnification is varied.
  • magnification is fixed.
  • scanning time or speed is varied. Since the fixing of the magnification fixes the scanning width, this increases the scanning velocity. Varying the scanning time or speed will produce a similar effect to varying the magnification.
  • the result will be that a series of "optimum" outputs will be obtained, corresponding to optimum focussing, for different magnifications or scanning times or speeds.
  • the optimum output at one magnification, scanning time or speed is compared with the "optimum" output derived for the previous scanning time. Provided the former "optimum" value is more “optimum” than the latter, the rest of the invention continues with a further magnification or scanning time or speed. If, however, the former "optimum" output is worse than the latter optimum output, then the system knows that it has passed the true optimum, and that true optimum can subsequently be used.
  • a memory stores the "optimum" output for each magnification or scanning time. Then, after all the magnifications or scanning times in a range of such magnifications or scanning times or speeds have been investigated, the most optimum output can be determined.
  • the present invention is particularly applicable to an electron microscope, in which case the focussing current is the current which determines the focussing of the condenser lens or condenser lenses forming an condenser lens array.
  • Fig. 1 is a schematic block diagram of part of an electron microscope, in which the present invention may be incorporated, an electron beam 1 is focussed on the surface of a specimen 4 by an electromagnetic condenser lens 3. As this occurs, the beam 1 is caused to scan over the surface of the specimen 4 due to the influence of a deflecting coil 2.
  • the signal (ordinarily, a secondary electron signal) 5 generated from the specimen 4 by irradiation of the electron beam 1 is received by a detector 6 and converted to an electric signal, and displayed on a CRT (VDU) (not shown) via a video amplifier 7 which generates a video signal.
  • VDU CRT
  • the magnification of the displayed image is equal to the ratio of the scanning width of the image displayed on the CRT to the scanning width of the electron beam over the specimen 4.
  • the diameter of the electron beam spot on the specimen 4 is minimized.
  • the spot diameter of the irradiated electron beam 1 on the specimen 4 is not minimized by manually changing the excitation current of the condenser lens 3. Instead of minimization by manually changing the focal length of the condenser lens 3, the device itself automatically accomplishes this.
  • the video output of the video amplifier 7 is fed to a differential circuit 8 which generates the differential of that output, and the modulus (absolute value) of that differential. Then, that differential is passed to an integration circuit 9 which determines the integral over a predetermined period of time of the modulus of the differential. It is the output of integration circuit 9 which indicates the optimum focussing.
  • the output of the integration circuit 9 is fed to a condenser lens current control circuit 10, which outputs a signal to a D/A converter 11 and hence via a current amplifier 12 to the condenser lens 3.
  • a deflecting coil current control circuit 13 for controlling the deflecting coil 2, and a start switch circuit 14 which starts the automatic focussing operation.
  • the condenser lens current control circuit 10, the deflecting coil current control circuit 13, and the starting switch circuit 14 are connected via a CPU bus 15 to a processing unit (CPU) 16 which controls the automatic focussing method.
  • a memory 17 may store current values for use by the deflecting coil current control circuit and there may be an operator controllable input 18 for inputting a magnification and/or a focussing range.
  • the excitation current of the condenser lens 3 is automatically searched for so as to maximize the high frequency components of the video signal.
  • the fundamental idea underlying this procedure is shown in Fig. 2.
  • the condenser lens current I0 stepwise with time t, from a minimum value I0MIN to a maximum value I0MAX (or conversely)
  • the optimum condenser lens current I0OP is automatically searched for, so as to maximize the absolute differential value
  • of the video signal S(I0) has a changing momentary value so that it is extremely difficult to process, due to the influence of noise or S/N ratio.
  • the video signal from the video amplifier 7, which has a band path filter is changed to an absolute differential value by the differential circuit 8 which generates an absolute value output, which is integrated by the integration circuit 9, over a predetermined time interval.
  • This value (the discrimination value) is determined as a function of the excitation current of the condenser lens 3, and the condenser lens current is automatically controlled using the D/A converter 11 and the current amplifier 12, so as to maximize the discrimination value.
  • the objective lens current control circuit 10 is used, which is connected to the CPU bus 15 and to the CPU 16 containing a suitable operation program. Automatic focusing is started by the automatic focusing start switch circuit 14.
  • the Start/End control for the differential circuit 8 and the integration circuit 9 is triggered by a CPU command transmitted via the CPU bus 15, which is coupled with the condenser lens current control effected through the condenser lens current control circuit 10.
  • the automatic focussing operation works well only within a high magnification domain, and does not work well within a low magnification domain. This is because the scanning time of the electron beam 1 over the specimen 4 is constant. Therefore, if the scanning width is large (that is, if magnification is low), the effective scanning speed over the specimen increased, and then the video signal frequency component corresponding to the optimum objective lens current could not be detected as the maximum value of the discrimination value ⁇
  • This invention proposes that the scanning electron microscope, or similar apparatus, has an automatic focusing function to overcome these said defects.
  • the control system of the deflecting coil current control circuit 13 which controls the magnification with respect to the deflecting coil 2 that is, the electron beam scanning width over the specimen 4) is included in an algorithm for automatic focussing, the problem underlying the present invention may be resolved.
  • the video signal is a function S(I0,M) of both of the conductor lens current I0 and the magnification M.
  • magnification is fixed at a predetermined value, as in a conventional automatic focussing system, then the maximum value of the discrimination value ⁇
  • the condenser lens current I0 is then fixed at a value (actually, a provisional optimum value I0OP(i)) which corresponds to the maximum value obtained as discussed.
  • the magnification M is changed so that a maximum discrimination value is obtained as shown in Fig. 3.
  • the search of the maximum discrimination value may be accomplished effectively and in a short time. For instance, if automatic focusing is carried out on a 1 ⁇ m wide photo resist pattern formed on a silicon-wafer, and if the scanning electron microscope which uses an electric field emission electron gun is used, 20000 times, 30000 times, and 40000 times are enough for the variable width and steps of the magnification. In any case, it is desirable that the device is constructed so as to make is possible to choose many kinds of variable width and steps of the magnification.
  • the magnification at the optimum magnification thus obtained is fixed and automatic focusing is carried out.
  • the true optimum value I0 may be determined at optimum magnification m OP in Fig3.
  • the condenser lens current is fixed on this value.
  • Fig. 4 illustrates steps in an automatic focussing method according to a first embodiment of the present invention.
  • operation starts at a first step 20, and the first stage in the method is to set a magnification M to an initial value M i , at step 21.
  • the current I0 for controlling the condenser lens 3 is set to an initial value I0 j at step 22.
  • the electron beam 1 is scanned on a sample 4 so that at step 23, the absolute value of the integration of the video current S is determined, and this is integrated at step 24 to generate a first output Q ji .
  • step 25 a comparison operation is carried out.
  • step 26 in which the value of j is increased by 1, to step 27, and this new value of j is then used for a new value of I0.
  • step 24 will then produce a new value of Q and this new value of Q can be compared with the first value of Q. If there is an increase in the value of Q then processing again passes via step 27. Thus, there will be iteration of steps 22, 23, 24, 25, and 27 until a value of Q is obtained which is less than or equal to the previous value.
  • step 30 is a further comparison step If the magnification M i is the first magnification used then processing immediately passes from step 30 to step 31 in which the value of i is increased by 1. For subsequent values of i the value of Q obtained for I0 OP(i) at magnification M i can be compared with the corresponding value of Q for the previous magnification value. If the new value of Q is greater than the previou value, again processing passes from step 30 to step 31. Thus, there will be a further iteration loop through steps 29, 30, 31, 21, and each of these iteration loops has a sub iteration loop of steps 22, 23, 24, 25 and 27.
  • Figure 5 shows a second embodiment of a method according to the present invention.
  • This method differs from that of Fig. 4 in that there is a range of magnifications, and measurement is carried out for all of those magnifications.
  • i is set to one below the minimum value at step 51, and then to the minimum at step 52.
  • This minimum value at step 52 generates a first magnification value at step 53.
  • the video current is investigated for a range of objective lens currents. This is the same operation as was carried out in Fig. 4, and the same reference numerals are used, being steps 22, 23, 24, 25, 27 and 28.
  • step 54 the optimum values of Q I0 and the corresponding magnification are stored in a memory.
  • step 55 checks if the maximum magnification has been reached If it has not, processing returns to step 52, to increment the magnification by one at step 53, and iteration then occurs. This iteration is repeated until the maximum value of the magnification is reached. At this point, a series of values of Q and corresponding I0 have been obtained for each magnification.
  • step 56 i is reset to the value at step 51, and then incremented by one as at step 52, shown by step 57.
  • the corresponding magnification and value of Q are then obtained at step 58, and the value of Q investigated at step 59.
  • step 59 will involve comparison with the immediately preceding value from the series of iterations through steps 57, 58, and 59.
  • step 59 is operating a similar way to step 30.
  • the start of the automatic focussing may manually initiate the starting switch circuit 14, to cause automatic focussing as in the embodiment of the present invention discussed above with reference to Fig. 1, or may automatically initiate after the operator moved a field for the microscopic inspection.
  • the movement of the field for the microscopic inspection may be performed on the basis of a pre-programmed command.
  • efficient automatic focusing may be carried out independently of the fineness of the specimen surface structure and the magnification which is set for the image observation, the automatic focusing is carried out surely and effectively.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Electron Sources, Ion Sources (AREA)
  • Automatic Focus Adjustment (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)
EP90301563A 1989-03-10 1990-02-14 Focussing an electron beam Expired - Lifetime EP0386894B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP1056476A JPH0766770B2 (ja) 1989-03-10 1989-03-10 電子線照射装置
JP56476/89 1989-03-10

Publications (3)

Publication Number Publication Date
EP0386894A2 EP0386894A2 (en) 1990-09-12
EP0386894A3 EP0386894A3 (en) 1991-04-24
EP0386894B1 true EP0386894B1 (en) 1994-08-31

Family

ID=13028156

Family Applications (1)

Application Number Title Priority Date Filing Date
EP90301563A Expired - Lifetime EP0386894B1 (en) 1989-03-10 1990-02-14 Focussing an electron beam

Country Status (4)

Country Link
US (1) US5032725A (ja)
EP (1) EP0386894B1 (ja)
JP (1) JPH0766770B2 (ja)
DE (1) DE69011908T2 (ja)

Families Citing this family (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0689687A (ja) * 1990-12-27 1994-03-29 Jeol Ltd 走査電子顕微鏡における自動焦点合わせ装置
JPH0594798A (ja) * 1991-05-21 1993-04-16 Jeol Ltd 焦点深度切り換え可能な電子顕微鏡等の電子光学観察装置
DE69224506T2 (de) * 1991-11-27 1998-10-01 Hitachi Instruments Eng Elektronenstrahlgerät
JP2535695B2 (ja) * 1992-01-13 1996-09-18 株式会社東芝 走査型電子顕微鏡の自動焦点合わせ方法
JP3109785B2 (ja) * 1993-12-21 2000-11-20 株式会社日立製作所 走査電子顕微鏡の自動焦点合わせ装置
US6278114B1 (en) 1997-12-19 2001-08-21 Kabushiki Kaisha Toshiba Method and apparatus for measuring dimensions of a feature of a specimen
JP2000048756A (ja) * 1998-07-27 2000-02-18 Seiko Instruments Inc 荷電粒子ビーム光学系の調整を行う方法およびその装置
US6521891B1 (en) * 1999-09-03 2003-02-18 Applied Materials, Inc. Focusing method and system
KR102268019B1 (ko) * 2019-04-30 2021-06-22 (주)코셈 인공 지능 학습 데이터를 활용한 전자 현미경

Family Cites Families (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5275261A (en) * 1975-12-19 1977-06-24 Jeol Ltd Test piece image dispaly unit
GB1585158A (en) * 1976-07-30 1981-02-25 Cambridge Scientific Instr Ltd Automatic focusing systems
JPS5457949A (en) * 1977-10-18 1979-05-10 Jeol Ltd Automatic focusing unit for scanning electron microscope and so on
JPS5569947A (en) * 1978-11-21 1980-05-27 Toshiba Corp Display unit
JPS5750756A (en) * 1980-09-12 1982-03-25 Jeol Ltd Objective lens current control method for scan type electron microscope
JPS585956A (ja) * 1981-07-01 1983-01-13 Hitachi Ltd 電子顕微鏡
JPS5825050A (ja) * 1981-07-03 1983-02-15 Jeol Ltd 電子線装置の焦点合わせ方法
JPS5814460A (ja) * 1981-07-17 1983-01-27 Internatl Precision Inc 電子顕微鏡の焦点合わせ方法及びこの方法を応用した像撮影方法及び装置
GB2118009B (en) * 1982-03-02 1985-09-04 Cambridge Instr Ltd Improvements in and relating to electron beam focussing

Also Published As

Publication number Publication date
DE69011908D1 (de) 1994-10-06
EP0386894A3 (en) 1991-04-24
US5032725A (en) 1991-07-16
DE69011908T2 (de) 1995-03-23
EP0386894A2 (en) 1990-09-12
JPH0766770B2 (ja) 1995-07-19
JPH02236937A (ja) 1990-09-19

Similar Documents

Publication Publication Date Title
EP0458418B1 (en) Method and apparatus for measuring internal defects
US5093572A (en) Scanning electron microscope for observation of cross section and method of observing cross section employing the same
US6172365B1 (en) Electron beam inspection method and apparatus and semiconductor manufacturing method and its manufacturing line utilizing the same
US7928376B2 (en) Element mapping unit, scanning transmission electron microscope, and element mapping method
US20030201393A1 (en) Electron microscope
US6548811B1 (en) Transmission electron microscope apparatus with equipment for inspecting defects in specimen and method of inspecting defects in specimen using transmission electron microscope
EP0386894B1 (en) Focussing an electron beam
US5144129A (en) Electron microscope
US7247849B1 (en) Automated focusing of electron image
US4199681A (en) Method and apparatus for automatically focusing an electron beam in a scanning beam device
US5130540A (en) Method and apparatus for automatic focusing of scanning electron microscope
JP2535695B2 (ja) 走査型電子顕微鏡の自動焦点合わせ方法
US6444967B1 (en) Microscope in-focus state detecting method and apparatus using image contrast detection
US6815677B2 (en) Scanning electron microscope and method of controlling the same
JP3021917B2 (ja) 電子ビーム装置における自動焦点合わせと非点収差補正方法
EP0735563A2 (en) Scanning electron microscope
JP4111908B2 (ja) 走査電子顕微鏡
EP0332140B1 (en) Focusing apparatus of electron microscope
EP0425204A2 (en) Secondary ion mass analyzing apparatus
JP3342580B2 (ja) 荷電粒子ビーム装置
JPH10154479A (ja) 合焦位置算出装置及びその方法
JP2633861B2 (ja) Ｓｅｍ式検査装置
JPH07161327A (ja) 荷電粒子ビーム装置における焦点合わせ方法
JPH0765771A (ja) 合焦位置検出装置
JPS63202835A (ja) 荷電ビ−ムの自動調整方法および自動調整装置

Legal Events

Date Code Title Description
PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

17P Request for examination filed

Effective date: 19900320

AK Designated contracting states

Kind code of ref document: A2

Designated state(s): DE GB

PUAL Search report despatched

Free format text: ORIGINAL CODE: 0009013

AK Designated contracting states

Kind code of ref document: A3

Designated state(s): DE GB

17Q First examination report despatched

Effective date: 19920429

GRAA (expected) grant

Free format text: ORIGINAL CODE: 0009210

AK Designated contracting states

Kind code of ref document: B1

Designated state(s): DE GB

REF Corresponds to:

Ref document number: 69011908

Country of ref document: DE

Date of ref document: 19941006

PLBE No opposition filed within time limit

Free format text: ORIGINAL CODE: 0009261

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: NO OPPOSITION FILED WITHIN TIME LIMIT

26N No opposition filed
REG Reference to a national code

Ref country code: GB

Ref legal event code: IF02

PGFP Annual fee paid to national office [announced via postgrant information from national office to epo]

Ref country code: GB

Payment date: 20050125

Year of fee payment: 16

PG25 Lapsed in a contracting state [announced via postgrant information from national office to epo]

Ref country code: GB

Free format text: LAPSE BECAUSE OF NON-PAYMENT OF DUE FEES

Effective date: 20060214

PGFP Annual fee paid to national office [announced via postgrant information from national office to epo]

Ref country code: DE

Payment date: 20060309

Year of fee payment: 17

GBPC Gb: european patent ceased through non-payment of renewal fee

Effective date: 20060214

PG25 Lapsed in a contracting state [announced via postgrant information from national office to epo]

Ref country code: DE

Free format text: LAPSE BECAUSE OF NON-PAYMENT OF DUE FEES

Effective date: 20070901